But stunned police on the U.S. side of the border say the law contradicts President Felipe Calderon's drug war, and some fear it could make Mexico a destination for drug-fueled spring breaks and tourism.

Tens of thousands of American college students flock to Cancun and Acapulco each year to party at beachside discos offering wet T-shirt contests and all-you-can-drink deals.

"Now they will go because they can get drugs," said San Diego Police Chief William Lansdowne. "For a country that has experienced thousands of deaths from warring drug cartels for many years, it defies logic why they would pass a law that will clearly encourage drug use."

Enacted last week, the Mexican law is part of a growing trend across Latin America to treat drug use as a public health problem and make room in overcrowded prisons for violent traffickers rather than small-time users.

Brazil and Uruguay have already eliminated jail time for people carrying small amounts of drugs for personal use, although possession is still considered a crime in Brazil. Argentina's Supreme Court ruled out prison for pot possession on Tuesday and officials say they plan to propose a law keeping drug consumers out of the justice system.

Colombia has decriminalized marijuana and cocaine for personal use, but kept penalties for other drugs.

Officials in those countries say they are not legalizing drugs — just drawing a line between users, dealers and traffickers amid a fierce drug war. Mexico's law toughens penalties for selling drugs even as it relaxes the law against using them.

"Latin America is disappointed with the results of the current drug policies and is exploring alternatives," said Ricardo Soberon, director of the Drug Research and Human Rights Center in Lima, Peru.

As Mexico ratcheted up its fight against cartels, drug use jumped more than 50 percent between 2002 and 2008, according to the government, and today prisons are filled with addicts, many under the age of 25.

Rojas has spent half his life snorting cocaine and sniffing paint thinner as he roamed Mexico City's streets in a daze. Most days he was roused awake by police demanding a bribe and forcing him to move along, he said.

"It's good they have this law so police don't grab you," said Rojas, whose name, I-V-A-N, is tattooed across his knuckles.

Rojas hit bottom three weeks ago when he could not score enough money for drugs by begging and found himself shaking uncontrollably. He accepted an offer for help from workers from a drug rehabilitation center who approached him on the street.

"Drugs were finishing me off," said Rojas, whose 13-year-old brother died of an overdose eight years ago. "I lost my brother. I lost my youth."

Juan Martin Perez, who runs Caracol, the nonprofit center helping Rojas, said the government has poured millions of dollars into the drug war but has done little to treat addicts. His group relies on grants from foundations.

The new law requires officials to encourage drug users to seek treatment in lieu of jail, but the government has not allocated more money for organizations like Caracol that are supposed to help them.

Treatment is mandatory for third-time offenders, but the law does not specify penalties for noncompliance.

"This was passed quickly and quietly but it's going to have to be adjusted to match reality," Perez said.

Supporters of the change point to Portugal, which removed jail terms for drug possession for personal use in 2001 and still has one of the lowest rates of cocaine use in Europe.

Portugal's law defines personal use as the equivalent of what one person would consume over 10 days. Police confiscate the drugs and the suspect must appear before a government commission, which reviews the person's drug consumption patterns. Users may be fined, sent for treatment or put on probation.

Foreigners caught with drugs still face arrest in Portugal, a measure to prevent drug tourism.

The same is not true for Mexico, where there is no jail time for anyone caught with roughly four marijuana cigarettes, four lines of cocaine, 50 milligrams of heroin, 40 milligrams of methamphetamine or 0.015 milligrams of LSD.

That's what concerns U.S. law enforcement at the border.

"It provides an officially sanctioned market for the consumption of the world's most dangerous drugs," San Diego County Sheriff Bill Gore said. "For the people of San Diego the risk is direct and lethal. There are those who will drive to Mexico to use drugs and return to the U.S. under their influence."

Don Thornhill, a retired Drug Enforcement Administration supervisor who investigated Mexican cartels for 25 years, said Mexico's rampant drug violence will likely deter most U.S. drug users, and the new law will allow Mexican police to focus on "the bigger fish."

The Bush administration criticized a similar bill proposed in Mexico in 2006, prompting then-President Vicente Fox to send it back to Congress. But Washington has stayed quiet this time, praising Calderon for his fight against drug cartels — a struggle that has seen some 11,000 people killed since Calderon took office in 2006.

"We work with Mexico every day to combat illegal drugs and cartel violence," State Department spokesman Ian Kelly said when asked about the law. "And we look forward to continuing that cooperation."